For me it depends on the finish- if covering something large with flat edges like a castle tower or big number I spread melted white choc on the rolled out sheet of clay first and lay the RKT on top then fold over to cover.

for sculpting I put it on directly, if my RKT turns out a little softer than I hoped I cover it roughly in melted chocolate first and let that set hard overnight. I find it easier to apply the clay in pieces and build up a shape gradually for this method.Hope that helps